Windows/Mac/Linux: InstantBird is an instant messaging client that looks great and won't drain system resources. If you want a highly customizable IM application that provides a consistent experience across Windows, Mac OS, and Linux, it's worth a try.

It's been a while since we last mentioned InstantBird, but the 1.0 release is a big improvement. The developers of InstantBird will be the first to tell you that it's not the most feature-rich application, but what it lacks in native features it makes up for in add-ons and themes contributed by the user community.

Windows/Mac/Linux (All platforms): There are tons of great instant messaging applications…
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The new version of InstantBird is inspired by the clean look of Pidgin (and actually uses their libpurple protocol library) but the UI is a bit more attractive. Plus, it's free. For alternatives, check out our picks for best instant messaging app for Mac and for Windows.